This achievement can only be done in Mount Magros, not the Super Volcano.

Progress towards this achievement can be checked by pausing the game and selecting "Achievements" from the pause menu. Hover the mouse over the achievement for a reminder of the description and a progress indicator.

One way to do this is to buy climbing hooks and a predug shaft. Go down the shaft carefully with the hooks, and whenever you see a myth dig about five squares above its head and slowly descend. When you can see its chamber carefully cause a cave in on its head, but beware of it exploding! Then take the dead body and continue. Another way to get the achievement is to buy the lantern (for seeing into the myth lair), sannox 5001, rock blaster and a gas mask or an aqualung (try avoiding gas and water tough). Keep digging down until you find a myth. Look into it's lair with the lantern. Then go away and dig to the level of the myth. Dig several blocks back and keep firing the rock blaster (with as few shot as possible as an erruption will mean the end of the game) until it's dead. Then loot the corpse. If you are a beginner try this, as this does not require to risk your life by going near the myths and kill them with rocks, but it's most likely to trigger an erruption before killing the 5th one. Advanced players should try using the rock dropping and using the blaster as the last resort (when the myth is after you or jumping at you)